Hydrogen and oxygen combine to form water vapour.
When 40 litres of hydrogen burn, how many litres of oxygen are used?

Explanation

To solve the problem of how many liters of oxygen are used when 40 liters of hydrogen burn to form water vapor, we can follow these steps:

### Step-by-Step Solution:

1. **Write the Balanced Chemical Equation**:
The reaction between hydrogen and oxygen to form water vapor can be represented by the balanced equation:
\[
2H_2(g) + O_2(g) \rightarrow 2H_2O(g)
...